There is a calculated escalation to be seen in the plagues. One can see patterns of two and patterns of three.
If you take the plagues in groups of two, you will notice that the first have to do with the Nile. God strikes the river of the land of the Nile, starting with its most obvious feature and that on which it most depends. He turns the water to blood and all the fish die, and then he brings frogs from the river. Horrible as these plagues are, they are only annoyances compared to what follows.
The next two plagues have to do with insects: the lice and flies. First the water, now the multiplied airborne insects, what is next? Epidemics, that’s what. The fifth plague is the disease of the cattle, the sixth is boils, on man and beast. Both have this in common, they are medical epidemics.
What is next? The stupendous hail and the locusts. God is targeting their crops in these. First the water went bad, then insects, next disease was rampant, and now their food supply is drastically depleted. What can be next?
The last two are the awful, sinister darkness during which nothing happens, and then the angel of death’s visitation at midnight, that most targeted of all the plagues. These are sinister, constituting a kind of psychological warfare. God begins gently enough, but with real warning, and escalates through each pair.
The other way to look at the plagues is in triads. Triads are the natural grouping that the pattern in which each plague is announced suggests. Moses is told to meet Pharaoh early in the morning by the river, or early in the morning, implying by the river, in the first, fourth and seventh plagues. The second, fifth, and eight plague are done by going into Pharaoh, presumably in his court. The third, sixth, and most dreadfully the ninth plague come unannounced. The pattern suggests God wants us to group them that way. What do we get if we do?
At the opening of each triad there is an escalation. You can see at the end of the third triad that the magicians warn about the finger of god. The fourth plague is when God first uses his finger to trace a line between Israel and Egypt (with flies, no less). The first triad are annoyances, but the second triad will go beyond it: that is when the epidemics spread. The last triad is introduced with the most narrated plague, and the most spectacular: the hail and fire. It is a warning of worse to come.
I think one of the points of it all is that the tenth plague is going to be correspondingly awful, like having another triad all in one. Another point is that God is doing two things simultaneously: giving Pharaoh a warning that starts relatively easily, though ominously, and gets sterner and sterner; while at the same time he is hardening his heat, by providing the scraps of excuse the perverse heart of Pharaoh searches in order to refuse the reality he is facing. There is something very terrible in knowing that the third plague comes unannounced when suddenly the unexpected darkness comes on Egypt.
God’s terrible plagues accomplish at least 5 things: (1) they relax Egypt’s grip long enough for Israel to go; (2) had God just smitten Egypt wholesale outright, Israel would never have attempted their emigration, since they could have just occupied Egypt—so they also create in Israel an expectation of departure; (3) the plundering of the Egyptians is possible when Egypt envisions further escalations if Israel doesn’t leave; (4) Moses acquires some stature as a leader, growing in his own confidence and in the people’s confidence in him; (5) Egypt has been reduced, and once its military power is destroyed also, it will be in no position to flex its muscle at neighboring states.
God went to war. When finally the irresistible army of Egypt was drowned, the people saw it. They realized the whole thing had been structured and were overawed at the scale on which God works. And the whole thing is a pattern of a greater deliverance which is coming and in which God’s enemies will be overcome with utter, apocalyptic finality.